Integrated circuits are very complex devices that include multiple layers. Each layer may include conductive material and/or isolating material while other layers may include semi-conductive materials. These various materials are arranged in patterns, usually in accordance with the expected functionality of the integrated circuit. The patterns also reflect the manufacturing process of the integrated circuits.
Integrated circuits are manufactured by complex multi-staged manufacturing processes. This process may include depositing resistive material on a substrate or layer, selectively exposing the resistive material by a photolithographic process, and developing the resistive material to produce a pattern that defines some areas to be later etched or otherwise processed. After the pattern is processed various materials, such as copper are disposed. The deposition step is usually followed by a removing access material. Copper is polished by mechanical means, while other materials can be polished by chemical processes and/or a combination of chemical as well as mechanical processes. The polishing can result in various deformation, such as dishing and erosion.
Copper is sensitive to oxidation, therefore, some of the manufacturers protects the copper layer by a dedicated protective layer that is deposited over copper, after the copper was deposited and/or polished. Usually this layer is made of materials such as Silicon Nitride and Silicon Carbide.
By applying the protection layer various manufacturing process can be executed in a more relaxed manner.
Various metrology, inspection and failure analysis techniques evolved for inspecting integrated circuits both during the manufacturing stages, between consecutive manufacturing stages, either in combination with the manufacturing process (also termed “in line” inspection techniques) or not (also termed “off line” inspection techniques). It is known that manufacturing failures may affect the electrical characteristics of the integrated circuits. Some of these failures result from unwanted deviations from the required dimensions of the patterns.
Electron beam metrology and defect detection tools, such as Scanning Electron Microscopes are used for high resolution measurement of surface features as well as surface defects and contaminations. These tools generate a spot of electrons that is very small. Typical spots may have a diameter of about few nanometers. The electron beam spot interacts with the surface and with a certain volume that is positioned near to the surface.
A typical protective layer is much thicker then the depth of the effective information volume of a typical Scanning Electron Microscope. For example, a protective layer can be few tens nanometers (typically 50 to 100 nm), while the depth of the effective information volume of a typical SEM is about few nanometers.
Accordingly, the typical Scanning Electron Microscope can not view defects that are below the protective layer.
There is a need to provide methods and systems for imaging of sub-surface layers, detecting hidden defects, and especially defects positioned below a protective layer.